A piezoelectric plastic film has flexibility and bendability that piezoelectric ceramic does not have. Particularly, a piezoelectric fluororesin film also has excellent properties such as heat resistance, wear resistance and chemical resistance. Therefore, the piezoelectric fluororesin film is a promising material for a piezoelectric element.
A piezoelectrically-treated polyvinylidene fluoride (PVDF) film is commonly known as the piezoelectric fluororesin film. Generally, a beta-type crystal of PVDF can be produced by stretching. Since this beta-type crystal has a polarity, the piezoelectricity can be generated by performing polarization treatment to orient molecular dipoles to the same direction.
Japanese Patent Laying-Open No. 60-055034 (PTD 1), for example, discloses fabrication of a piezoelectric element by using a method including the steps of: monoaxially stretching an unoriented sheet of approximately 100 μm in thickness obtained by melt extrusion molding of PVDF; vacuum-depositing metal on both surfaces of the monoaxially-stretched film to form an electrode; and applying a DC high electric field of approximately 1000 kV/cm in the film thickness direction for 60 minutes, while performing heating at a temperature equal to or lower than the melting point of the film.
However, according to the method described in PTD 1, a high voltage and application of the voltage for a long time are required to provide the piezoelectricity, and in addition, the obtained piezoelectricity is insufficient. Moreover, if pores are present in the film, air discharge or breakdown occurs during polarization treatment. Therefore, it becomes difficult to achieve application of a high voltage and further uniform application of an electric field. As a result, it is conceivable that sufficient piezoelectricity is not produced.
Under such circumstances, various methods for enhancing the piezoelectricity of the piezoelectric fluororesin film have been proposed.
Japanese Patent Laying-Open No. 06-342947 (PTD 2), for example, proposes performing polarization treatment on a PVDF film, with pores of the porous PVDF film impregnated with an insulation oil and with the PVDF film sandwiched between a pair of dielectric sheets.
Specifically, Example 1 of PTD 2 describes that a solution of vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymer is casted on a glass plate and dried to form a porous film (porosity: 70%, average pore size: 0.45 μm) of a communication pore type having a film thickness of 130 μm, and the porous film is sandwiched between PVDF-based monoaxially-stretched sheets, and polarization treatment is performed on the porous film by corona charging. Example 2 describes that the porous film in Example 1 is impregnated with the insulation oil and polarization treatment is performed on the porous film in the same manner as described above. PTD 2 also describes that the piezoelectric property (an amount of increase in electric charges with respect to pressure increase) of the piezoelectric porous films obtained in Examples 1 and 2 is higher than that obtained in the case of corona charging of the porous film alone (Comparative Examples).
In addition, Japanese National Patent Publication No. 2009-501826 (PTD 3) proposes applying pressure, under heating, to a beta-phase porous PVDF film obtained from a solution having PVDF dissolved in dimethylformamide (DMF) or dimethylacetamide (DMA), thereby crushing pores. According to this method, the piezoelectricity is enhanced by crushing the pores and transforming the PVDF film into a substantially beta-phase nonporous film.
As described above, in the PVDF-based film, an attempt to enhance the piezoelectricity is made by increasing a ratio of the beta-type crystal portion that produces the piezoelectricity or preventing air discharge that impairs the effect of polarization treatment. However, the effect of enhancing the piezoelectricity is insufficient. In addition, heating causes the beta-type crystal of PVDF to return to the alpha type that does not have the piezoelectricity. Therefore, the PVDF-based film is not satisfactory in terms of heat resistance as well.
As a piezoelectric plastic film that produces the piezoelectricity by a mechanism totally different from a PVDF film that produces the piezoelectricity due to its molecular structure and crystal structure, U.S. Pat. No. 4,654,546 (PTD 4) proposes a stretched porous polypropylene film having disc-like bubbles.
In recent years, this porous polypropylene film has been commercially available as an Emfit (registered trademark) ferroelectret film from Emfit, Ltd. and has received attention because this film exhibits a high piezoelectric modulus. This Emfit (registered trademark) film is a film having a lamellar structure with many flat pores, which is formed by biaxially stretching a porous polypropylene film and further injecting a high-pressure gas to expand the pores in the film [http://www.emfit.com/en/sensors/products_sensors/emfit-film/ (NPD 1, homepage of Emfit, Ltd.)]. When corona discharge of such a film is performed, positive and negative electric charges are trapped in upper and lower surfaces of the pores and the film has the piezoelectricity. There is also a report that a piezoelectric constant d33 of the Emfit (registered trademark) film is several tens times as large as that of the PVDF film [http://www.europrotech.com/Euro/trade/t_emfit2.html (NPD 2, homepage of Europrotech LLC, especially Table 1)].
In addition, as described in Masatoshi Nakayama, et al., “Piezoelectricity of Ferroelectret Porous Polyethylene Thin Film”, Japanese Journal of Applied Physics 48 (2009) (NPD 3), it is reported that piezoelectric constant d33 of a piezoelectric film obtained by corona discharge of a ferroelectret film having a thickness of 30 μm and a porosity of 58% and made of porous polyethylene (Fp-PE) is three times as large as that of the PVDF film.
Generation of the piezoelectricity of porous polypropylene and porous polyethylene is based on electrical charging of micron-size to submillimeter-size pores, which is totally different from production of the piezoelectricity based on a dipole due to the nano-size molecular structure and crystal structure of PVDF.
As a piezoelectric element made of a porous fluorine-based resin that produces the piezoelectricity based on electrical charging of pores, Japanese Patent Laying-Open No. 2007-231077 (PTD 5), for example, proposes a piezoelectric element fabricated by mixing a foaming agent into tetrafluoroethylene-hexafluoropropylene copolymer (FEP) and foaming the copolymer to form a sheet (thickness: 200 μm, foaming rate: 40%) having closed pores, and causing electric charges to be trapped in this sheet by using a corona discharge device. PTD 5 describes that this piezoelectric element exhibits a quasi-static piezoelectric constant d33 larger than that of a piezoelectric element fabricated by corona discharge of a nonporous fluororesin film in the same manner.
As described above, higher piezoelectricity may be obtained than that of the PVDF film that produces the piezoelectricity based on the dipole due to the nano-size molecular structure and crystal structure. Therefore, a method for enhancing the piezoelectricity by using a porous plastic film other than the PVDF film has been under study in recent years.
As an electret having high piezoelectricity comparable to that of an inorganic piezoelectric material and formed of a polymer porous body excellent in workability, Japanese Patent Laying-Open No. 2010-186960 (PTD 6), for example, describes an electret in which “an average aspect ratio of a pore is 7 or more and 30 or less, the average number of pores in a thickness direction is 1 or more and 10 or less, and an average pore diameter in the thickness direction is 30 μm or larger and 200 μm or smaller” (claim 1). A polypropylene foam obtained by biaxially stretching an organic polymer foam is used as the polymer porous body (Examples). PTD 6 describes in paragraph 0011 that by forming a pore having a large aspect ratio, the pore diameter is increased and the piezoelectric performance comparable to that of an inorganic piezoelectric body is obtained. In addition, an average value of diameters in the thickness direction obtained by observing a cross section cut in parallel to the stretching direction with a scanning electron microscope is used as the pore diameter (paragraph 0026).
In addition, Japanese Patent Laying-Open No. 2011-018897 (PTD 7) and Japanese Patent Laying-Open No. 2011-210865 (PTD 8) propose a porous resin sheet for a piezoelectric element which has a bubble having an average maximum vertical chord length of 1 to 40 μm and an average aspect ratio (average maximum horizontal chord length/average maximum vertical chord length) of 0.7 to 4.0, and which has a volumetric porosity of 20 to 75%. Such a porous resin sheet is manufactured by mixing a resin forming a plastic film with a phase separation agent to fabricate a sheet having a sea-island structure in which the phase separation agent is an island, curing the resin component, and thereafter, removing the island of the phase separation agent. Polyetherimide and annular olefin polymer are used as the resin component (Examples).
An object of the invention disclosed in PTD 7 is to provide the porous resin sheet for a piezoelectric element having a high piezoelectric modulus and a high compressive stress. PTD 7 describes in paragraph 0013 that this object can be achieved by increasing the size of a bubble forming a dipole to increase an amount of change in the dipole, and decreasing the aspect ratio to adjust an elastic modulus in the thickness direction. PTD 7 also describes in paragraph 0014 that when the average maximum vertical chord length exceeds 40 μm, the voltage density applied to bubbles during electrical charging treatment becomes lower and spark discharge becomes less likely to occur. PTD 7 discloses in Table 1 that piezoelectric constant d33 of a piezoelectric film including a porous resin film (polyetherimide, cycloolefin copolymer, polystyrene) having an average maximum vertical chord length of 2.63 μm to 4.80 μm is 66 to 1449 pC/N.
Along with the recent widespread use of electronic terminals such as a touch panel, use of a piezoelectric element formed of a plastic film in the touch panel and the like has also been under study. High transparency is desired for the piezoelectric element formed of the plastic film which is used in such an application.